The present invention generally relates to a filter device and more particularly, to an electrical filter device such as an electrical filter, transmit-receive branching filter (a so called dupler) or the like which employs at least two dielectric coaxial resonators each including a dielectric member provided between an outer conductor and an inner conductor, and which may be used, for example, in a transmission and reception system for a radio communication equipment.
Commonly, electrical filter devices constituted by coaxial resonators each having a dielectric member disposed between an outer and an inner conductors (referred to merely as dielectric coaxial resonators hereinbelow) may be broadly divided into two types, one type of which disclosed, for example, in Japanese Laid Open Patent Application Tokkaisho No. 56-57302 has the construction shown schematically in FIG. 1, while the other type of which disclosed, for example, in Japanese Laid Open Patent Application Tokkaisho No. 56-57304 has the construction shown schematically in FIG. 2.
In the conventional arrangement as shown in FIG. 1, since inner conductors of respective 1/4 wavelength dielectric coaxial resonators R are coupled, to each other by coupling capacitors C which are disposed between the inner conductors of respective neighboring dielectric coaxial resonators R, it is necessary to provide a cut-off space S at the open end side (where voltage distributed within the resonator has a maximum reduction in current) of each of the dielectric coaxial resonators R.
In the above arrangement, the capacitors C and lines connecting said capacitors C to each other form an imaginary signal propagation passage P as shown by two-dotted chain lines in the drawings. The dimensions and configurations of the cut-off space S referred to above are chosen to avoid coupling between the neighboring dielectric coaxial resonators R by electromagnetic waves, and therefore, the respective neighboring coaxial resonators R are coupled to each other only by the capacitors C. Nevertheless, there are cases where unnecessary coupling takes place due to leakage signal components propagated along inner surface of a casing (not shown). Moreover, each cut-off space S is inevitably required for one dielectric coaxial resonator R, thus making it difficult to make a device having a against compact size.
Meanwhile, in the known arrangement as shown in FIG. 2, the direction of the signal propagation passage P is arranged to be aligned with the axial direction of 1/4 wavelength dielectric coaxial resonators R, while the open ends of the resonators R are coupled to each other by capacitors C, with short-circuit ends of the respective resonators R being magnetically coupled as shown. Although the cut-off spaces S are still required at the open end sides of the dielectric coaxial resonators R in a manner similar to that of the arrangement of FIG. 1, since one cut-off space S may be commonly used for the two confronting resonators R, the casing for the arrangement can be made compact by that extent, with a less possibility of occurrence of unnecessary coupling. However, in the case where an electrical filter is to be constructed, it becomes necessary to partly employ the induction (i.e. magnetic) coupling which is more complicated in structure than the capacitor coupling, and if the filter is entirely constructed by the capacitor coupling, it is required to partly employ 1/2 wavelength dielectric coaxial resonators, thus being inconsistent with the requirement for the compact size.